Programmable logic controllers (PLCs) known in the art may control industrial machines installed in factory production lines.
A PLC includes a central processing unit (CPU) for controlling the PLC, an input unit for receiving a signal from a sensor, and an output unit for outputting a control signal to a control target device such as an industrial machine. The CPU has memory for storing a user program. The user program can be edited with a tool device connected to the CPU.
The PLC repeatedly performs the processing including storing a signal input in the input unit into the memory of the CPU, executing a user program, writing the execution results (computational results) obtained from the user program into the memory and transmitting the results to the output unit, and performing peripheral processing, or transmitting and receiving data to and from the tool device. Through such processing, the PLC controls an industrial machine based on inputs from the sensor.
Programmable automation controllers (PACs) incorporating the sophisticated software capabilities of a personal computer and the reliability of a PLC have also been known (refer to, for example, Patent Literature 1).
A PAC described in Patent Literature 1 uses time-sharing to execute multiple tasks in parallel, with each task being executed in cycles preset for the task. In other words, the PAC performs processing in a multitasking manner. Execution of a user program is assigned to each task.